1. Field of the Invention
The present invention relates to an ejector for ejecting a molded product from a mold in an injection molding machine, and in particular to a method of and an apparatus for determining a separating force for separating the molded product from the mold by the ejector.
2. Description of the Related Art
There have been proposed several methods of determining a separating force for separating a molded product from a mold, and detecting status of the molded product and/or the mold and further errors in various settings of the ejector mechanism based on the determined separating force. For example, it is known from Japanese Patent Laid-Open Publication No. Heisei 10-119107 to detect a load exerted on ejector pins for preventing breakage of the ejector pins and/or a core of the mold by an abnormal load thereon by an interference of the ejector pins and the core because of an error of setting of projection amount of the ejector pin, and for detecting remaining of the molded product in the molds.
As means for detecting the separating force, it is generally known to measure the load exerted on the ejector mechanism directly using a sensor and by estimation of the load using a disturbance estimating observer incorporated in a controller of a servomotor for driving the ejector mechanism.
Japanese Patent Laid-Open Publication No. Showa 56-146713 describes a method of determining a separating force of a molded product as a difference between peak values of respective loads exerted on an ejector rod in driving an ejector mechanism with and without a molded product in the mold, which are detected by a strain sensor mounted on the ejector rod. In this method, the separating force is determined by subtracting the peak value of the load exerted on the ejector rod without a molded product in the mold, which is constituted by a resistance force in the mold, from the peak value of the load exerted on the ejector rod with a molded product in the mold, which is constituted by a sum of the inner resistance force and the separating force for separating the molded product from the mold. However, time at which the inner resistance force reaches its peak value is generally different from time at which the separating force excluding the inner resistance force reaches its peak value, and therefore the difference of the peak values of the loads with and without a molded product in the mold does not precisely represent the separating force. For example, in the case where the inner resistance force reaches the peak value at the end of the ejector motion and the separating force reaches the peak value at the beginning of the motion, the difference obtained by subtracting the peak value of the load without a molded product (detected at the end of the ejector motion) from the peak value of the load with a molded product does not represent the peak value of the separating force because the resistance force included in the peak value with a molded product is not subtracted from the peak value of the ejecting force with the molded product, to cause a problem of failing in precise determination of the separating force.
Further, in the above method of determining the separating force using a sensor mounted on the ejector pin, the sensor has to be incorporated in the mold to have a complex structure and raise a manufacturing cost, and also causing a problem of reliability of the sensor in mass production.
Japanese Patent Laid-Open Publication No. Showa 61-182920 describes a method of determining the separating force indirectly by measuring a time period of operation of the ejector mechanism, since the time period of ejecting a molded product form a mold varies in dependence on a resistance of the molded product in separating from the mold since adherence of the molded product on the mold differs in dependence on a filling pressure of resin material into a cavity of the mold. In this the method, in order to detect variation of the separating force, it is necessary to lower a driving force of the ejector to make the ejecting time vary with variation of the separating force, to render it difficult to establish a precise relationship between the measured time and the ejecting force, i.e., failing in performing stable ejection of a molded product or in precisely determining the separating force.
In the above-mentioned Japanese Patent Laid-Open Publication No. Heisei 10-119107, the disturbance load torque is estimated by a disturbance estimating observer incorporated in a servo controller for controlling a servomotor for driving the ejector mechanism, and an abnormal signal informing a collision of the ejector rod with an obstacle is issued when the estimated disturbance load torque continuously exceeds an allowable range for a predetermined time period. In this method, the estimated disturbance load torque includes frictions of movable components moving in the mold and an elastic force of return springs for returning an ejector plate in the mold, and therefore the separating force in separating the molded product from the mold is not detected with high precision.
The method of determining a separating force of a molded product from a mold in an injection molding machine of the present invention comprises the steps of driving an ejector mechanism in a molding operation without injection of resin not to produce a molded product in the mold, and obtaining and storing a load exerted on the ejector mechanism at every predetermined period or every predetermined position of the ejector mechanism in driving the ejector mechanism; driving an ejector mechanism in a molding operation with injection of resin to produce a molded product in the mold, and obtaining a load exerted on the ejector mechanism at every predetermined period or every predetermined position in driving the ejector mechanism; and obtaining a difference between the load obtained and stored in driving the ejector mechanism without molded product and the load obtained in driving the ejector mechanism with the molded product in the mold as a separating force of the molded product at every predetermined period or every predetermined position of the ejector mechanism.
The separating force may be obtained in a predetermined range in a motion stroke of the ejector mechanism.
The loads exerted on the ejector mechanism may be detected by means of a specific sensor or in the case where the ejector mechanism is driven by a servomotor, the loads may be obtained based on a driving force of the servomotor. In this case, the driving force of the servomotor may be obtained by means of a control program for controlling the servomotor.
Judgment on defectiveness/non-defectiveness of the molded product and abnormality of the ejector mechanism may be performed based on the determined separating force.
Further, the present invention provides an apparatus for determining a separating force of a molded product from a mold in an injection molding machine, including means for carrying out respective steps of the above method.